I can safely say that I have had a lifetime worth of shonen in the 18 years I've lived.
There's however only one that completly blew me away:
Air Gear!!
Full Metal Alchemist earns all my respect, due it's non-filler nature and great story. But DBZ earns credit as well, because I have always enjoyed this show during my childhood.
Me im new here but my favorite was the entire dragon ball series up till GT which even though was great in its own way...it did not live up to the standards of the last two and wasnt even based on the manga though i do want to say one thing ending to GT was a really really bad way to end off the series so i like to think it ended after the fight with buu
the best shonen :
http://a5.sphotos.ak.fbcdn.net/hphotos-ak-snc6/180605_187565614607798_187564784607881_519845_1365223_n.jpg
Shonan Junaï Gumi (Young GTO) ==> hilarous, great story... the best